Conventionally both natural and synthetic rubber are vulcanized in the presence of sulphur or peroxide at high temperature and pressure. This process has several significant shortcomings. Because of rubber's poor thermal conductivity, the process is slow and energy intensive. Typically several hours may be required to vulcanize thick sections of rubber at temperatures as high as 400.degree. F. and pressures of 2000 psi. Poor bonding between non-polar conventionally vulcanized rubber and polar reinforcements, such as cords, fabrics, and metal, often leads to interfacial failure. The additives that are used to accelerate the curing process and the special surface treatments that are applied to improve the bonding characteristics add to the manufacturing cost. The additives often cause scorching of the rubber compound during the molding stage.
The present invention largely overcomes the disadvantages of the conventional process. Poor thermal conductivity of rubber, a major problem in conventional thermal curing, becomes an advantage in the present ultrasonic process. In the conventional process poor conductivity resists penetration of the heat which is applied from the outside. In the present process, however, the ultrasonically-produced heat is generated internally, and poor conductivity acts to retain this heat and keep it from escaping. The internal generation of the vulcanizing heat uniformly throughout the material also eliminates the undesirable scorching that occurs when the heat is applied from the outside. The present method of ultrasonic vulcanization tends to change the interfacial property of rubber and the reinforcing materials to improve bonding. Improved wetting and flow characteristics produced by ultrasonic vulcanization have the potential to increase the interfacial bond strength between the rubber and currently used reinforcing materials.
We have successfully vulcanized specimens of natural and synthetic rubber with the application of ultrasonic energy at a significantly shorter time and lower total energy consumption and with a minimum of additives compared to the conventional thermal process. The production of many experimental samples has demonstrated the technique to be both reliable and repeatable.
The present invention provides several technical and economic benefits such as more than 100 percent increase in production rate due to shorter cure time; better than 50 percent energy savings; potential for uniform cure especially for thick sections; potential for improved bonding qualities resulting in better mechanical and aging properties of the final product; reduced raw material costs resulting from reduction or elimination of costly additives such as accelerators, activators, and coupling agents; lower mold temperature, eliminating the possibility of scorching in the mold; accelerated degassing; and potential for significant overall cost savings from all of the above benefits.